1、eventbus的使用

在activity的oncreate和ondestroy中注册和解除注册，在需要接收数据处理的方法上使用@subscribe注解，在发送数据出使用post。eventbus相比android内部广播优势在于，使用起来十分简洁，不需要像广播那种的注册，可携带数据量也比广播要多，广播仅可携带string、int等简单类型数据参数，eventbus可传递实体类，列表等数据。

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        EventBus.getDefault().register(this);
        setContentView(R.layout.activity_main);
        ...
    }
    
    @Subscribe(threadMode = ThreadMode.MAIN)
    public void getData(List<MeiLv> list) {
    ......
    }
    
    @Override
    protected void onDestroy() {
        EventBus.getDefault().unregister(this);
        super.onDestroy();
    }
    
    //发送数据到观察者
    EventBus.getDefault().post(object);

2、原理：

流程如下：

几个重要变量：

//初始化eventbus时候使用，初始化了一批判断条件参数
private static final EventBusBuilder DEFAULT_BUILDER = new EventBusBuilder();
//事件类型缓存列表
private static final Map<Class<?>, List<Class<?>>> eventTypesCache = new HashMap<>();
//用来保存注册的观察者列表
private final Map<Class<?>, CopyOnWriteArrayList<Subscription>> subscriptionsByEventType
//解析观察者的主要功能类
private final SubscriberMethodFinder subscriberMethodFinder;
//描述观察者方法
SubscriberMethod：
    final Method method;//方法名
    final ThreadMode threadMode;//线程类型，是后台线程还是UI线程
    final Class<?> eventType;//事件类型，也就是用于接收的参数类型
    final int priority;//优先级
    final boolean sticky;//是否注册就就马上发送一次消息
    /** Used for efficient comparison */
    String methodString;

注册流程如下：

public void register(Object subscriber) {
        Class<?> subscriberClass = subscriber.getClass();
        //解析类中带有的@subscribe的方法
        List<SubscriberMethod> subscriberMethods = subscriberMethodFinder.findSubscriberMethods(subscriberClass);
        synchronized (this) {
            for (SubscriberMethod subscriberMethod : subscriberMethods) {
                subscribe(subscriber, subscriberMethod);
            }
        }
    }
    
private void subscribe(Object subscriber, SubscriberMethod subscriberMethod) {
        Class<?> eventType = subscriberMethod.eventType;
        Subscription newSubscription = new Subscription(subscriber, subscriberMethod);
        CopyOnWriteArrayList<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
        //如果当前subscriptionsByEventType中没有包含该事件类型，就将其添加到该列表中，如果已经有了，则检查当前观察者是否已经添加在该事件类型的观察者列表中了，如果已经在了，则抛出异常，这样注册类中带有@subscribe注解的方法就都添加到了不同事件类型所对应的观察者列表中了
        if (subscriptions == null) {
            subscriptions = new CopyOnWriteArrayList<>();
            subscriptionsByEventType.put(eventType, subscriptions);
        } else {
            if (subscriptions.contains(newSubscription)) {
                throw new EventBusException("Subscriber " + subscriber.getClass() + " already registered to event "
                        + eventType);
            }
        }
        //按照优先级将新的观察者添加到该事件类型的列表中
        int size = subscriptions.size();
        for (int i = 0; i <= size; i++) {
            if (i == size || subscriberMethod.priority > subscriptions.get(i).subscriberMethod.priority) {
                subscriptions.add(i, newSubscription);
                break;
            }
        }

        List<Class<?>> subscribedEvents = typesBySubscriber.get(subscriber);
        if (subscribedEvents == null) {
            subscribedEvents = new ArrayList<>();
            typesBySubscriber.put(subscriber, subscribedEvents);
        }
        subscribedEvents.add(eventType);
        
        //如果该subscribemethos包含粘性属性，则继续发送事件通知观察者。
        if (subscriberMethod.sticky) {
            if (eventInheritance) {
                // Existing sticky events of all subclasses of eventType have to be considered.
                // Note: Iterating over all events may be inefficient with lots of sticky events,
                // thus data structure should be changed to allow a more efficient lookup
                // (e.g. an additional map storing sub classes of super classes: Class -> List<Class>).
                Set<Map.Entry<Class<?>, Object>> entries = stickyEvents.entrySet();
                for (Map.Entry<Class<?>, Object> entry : entries) {
                    Class<?> candidateEventType = entry.getKey();
                    if (eventType.isAssignableFrom(candidateEventType)) {
                        Object stickyEvent = entry.getValue();
                        checkPostStickyEventToSubscription(newSubscription, stickyEvent);
                    }
                }
            } else {
                Object stickyEvent = stickyEvents.get(eventType);
                checkPostStickyEventToSubscription(newSubscription, stickyEvent);
            }
        }
    }
    
    private void checkPostStickyEventToSubscription(Subscription newSubscription, Object stickyEvent) {
        if (stickyEvent != null) {
            // If the subscriber is trying to abort the event, it will fail (event is not tracked in posting state)
            // --> Strange corner case, which we don't take care of here.
            postToSubscription(newSubscription, stickyEvent, isMainThread());
        }
    }
    
    void invokeSubscriber(Subscription subscription, Object event) {
        //反射调用观察者method
        try {
            subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
        } catch (InvocationTargetException e) {
            handleSubscriberException(subscription, event, e.getCause());
        } catch (IllegalAccessException e) {
            throw new IllegalStateException("Unexpected exception", e);
        }
    }

注册流程：将类中带有@subscribe注解的方法分别添加到eventbus中的subscriptionsByEventType列表中，subscriptionsByEventType是按照事件类型来分类，也就是该方法的参数。

事件发送流程：
通过PostingThreadState来控制发送事件同步，首先将其加入到队列中eventQueue，如果当前正在发送则直接返回，正在发送的操作会将队列中的数据全部发送，发送操作主要是根据事件类型，将subscriptionsByEventType中保存的对应事件类型下的观察者列表中的所有观察者获取出来，之后反射该method方法。

public void post(Object event) {
        PostingThreadState postingState = currentPostingThreadState.get();
        List<Object> eventQueue = postingState.eventQueue;
        eventQueue.add(event);

        if (!postingState.isPosting) {
            postingState.isMainThread = isMainThread();
            postingState.isPosting = true;
            if (postingState.canceled) {
                throw new EventBusException("Internal error. Abort state was not reset");
            }
            try {
                while (!eventQueue.isEmpty()) {
                    postSingleEvent(eventQueue.remove(0), postingState);
                }
            } finally {
                postingState.isPosting = false;
                postingState.isMainThread = false;
            }
        }
    }
    
    private void postSingleEvent(Object event, PostingThreadState postingState) throws Error {
        Class<?> eventClass = event.getClass();
        boolean subscriptionFound = false;
        if (eventInheritance) {
            List<Class<?>> eventTypes = lookupAllEventTypes(eventClass);
            int countTypes = eventTypes.size();
            for (int h = 0; h < countTypes; h++) {
                Class<?> clazz = eventTypes.get(h);
                subscriptionFound |= postSingleEventForEventType(event, postingState, clazz);
            }
        } else {
            subscriptionFound = postSingleEventForEventType(event, postingState, eventClass);
        }
        if (!subscriptionFound) {
            if (logNoSubscriberMessages) {
                logger.log(Level.FINE, "No subscribers registered for event " + eventClass);
            }
            if (sendNoSubscriberEvent && eventClass != NoSubscriberEvent.class &&
                    eventClass != SubscriberExceptionEvent.class) {
                post(new NoSubscriberEvent(this, event));
            }
        }
    }
    
    private boolean postSingleEventForEventType(Object event, PostingThreadState postingState, Class<?> eventClass) {
        CopyOnWriteArrayList<Subscription> subscriptions;
        synchronized (this) {
        //根据事件类型从之前注册时候添加的列表：subscriptionsByEventType中将该事件类型的所有观察者获取到列表中
            subscriptions = subscriptionsByEventType.get(eventClass);
        }
        if (subscriptions != null && !subscriptions.isEmpty()) {
        //轮询观察者列表中的观察者，依次发送数据
            for (Subscription subscription : subscriptions) {
                postingState.event = event;
                postingState.subscription = subscription;
                boolean aborted = false;
                try {
                    postToSubscription(subscription, event, postingState.isMainThread);
                    aborted = postingState.canceled;
                } finally {
                    postingState.event = null;
                    postingState.subscription = null;
                    postingState.canceled = false;
                }
                if (aborted) {
                    break;
                }
            }
            return true;
        }
        return false;
    }
    
    private void postToSubscription(Subscription subscription, Object event, boolean isMainThread) {
        switch (subscription.subscriberMethod.threadMode) {
            case POSTING:
                invokeSubscriber(subscription, event);
                break;
            case MAIN:
                if (isMainThread) {
                    invokeSubscriber(subscription, event);
                } else {
                    mainThreadPoster.enqueue(subscription, event);
                }
                break;
            case MAIN_ORDERED:
                if (mainThreadPoster != null) {
                    mainThreadPoster.enqueue(subscription, event);
                } else {
                    // temporary: technically not correct as poster not decoupled from subscriber
                    invokeSubscriber(subscription, event);
                }
                break;
            case BACKGROUND:
                if (isMainThread) {
                    backgroundPoster.enqueue(subscription, event);
                } else {
                    invokeSubscriber(subscription, event);
                }
                break;
            case ASYNC:
                asyncPoster.enqueue(subscription, event);
                break;
            default:
                throw new IllegalStateException("Unknown thread mode: " + subscription.subscriberMethod.threadMode);
        }
    }
    
    void invokeSubscriber(Subscription subscription, Object event) {
        try {
            subscription.subscriberMethod.method.invoke(subscription.subscriber, event);
        } catch (InvocationTargetException e) {
            handleSubscriberException(subscription, event, e.getCause());
        } catch (IllegalAccessException e) {
            throw new IllegalStateException("Unexpected exception", e);
        }
    }

解除注册：

public synchronized void unregister(Object subscriber) {
        List<Class<?>> subscribedTypes = typesBySubscriber.get(subscriber);
        if (subscribedTypes != null) {
            for (Class<?> eventType : subscribedTypes) {
                unsubscribeByEventType(subscriber, eventType);
            }
            typesBySubscriber.remove(subscriber);
        } else {
            logger.log(Level.WARNING, "Subscriber to unregister was not registered before: " + subscriber.getClass());
        }
    }
    
    private void unsubscribeByEventType(Object subscriber, Class<?> eventType) {
        List<Subscription> subscriptions = subscriptionsByEventType.get(eventType);
        if (subscriptions != null) {
            int size = subscriptions.size();
            for (int i = 0; i < size; i++) {
                Subscription subscription = subscriptions.get(i);
                if (subscription.subscriber == subscriber) {
                    subscription.active = false;
                    subscriptions.remove(i);
                    i--;
                    size--;
                }
            }
        }
    }